Bumblebees imported from Europe infected with parasites pose a serious threat to the UK’s wild and honey bee populations, according to a new study.

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Each year, more than a million bumblebee colonies are imported by countries across the globe to pollinate a variety of crops, with the UK alone importing between 40,000 and 50,000. Although the colonies are said by the global suppliers to be disease free, a recent study in the Journal of Applied Ecology has found that more than three-quarters of those imported into the UK each year are riddled with parasites.

Scientists purchased 48 commercially produced bumblebee colonies, each containing up to 100 bees, from three different European suppliers during 2011 and 2012, and screened each one for parasite DNA. The results showed that an alarming 77% of the colonies were infected with parasites, a situation which has serious implications for the health of the UK’s native wild bees and honey bees, many of which are already suffering severe declines.

"These parasites will undoubtedly be spilling over into wild and honey bees and very probably having negative effects on them," said lead researcher Professor William Hughes, from the University of Sussex. "It is no great leap to think that damage is already being done."

The results of the study revealed that the imported bumblebee colonies carried several different parasites, five of which were found in the bees themselves and three in the pollen provided by the suppliers as food. These parasites included three main bumblebee parasites (Crithidia bombi, Nosema bombi and Apicystis bombi), three honey bee parasites (Nosema apis, Ascosphaera apis and Paenibacillus larvae), and two which infect both bumblebees and honey bees (Nosema ceranae and deformed wing virus).

With the decline of pollinating insects in the UK in recent years, food producers are becoming increasingly reliant upon imported bees.

"Over a million colonies are imported globally — it's a huge trade," said Professor Hughes. "A surprisingly large number of these are produced in factories, mainly in Eastern Europe. We couldn't grow tomatoes in this country without these bumblebees. We sought to answer the big question of whether colonies that are being produced now have parasites and, if so, whether those parasites are actually infectious or harmful."

The potential impacts on native wild bee and honey bee species could be extremely severe, from harming the bees' ability to learn, which is vital for finding food, to killing them outright.